Mechanisms of IL‐8 suppression by Treponema denticola in gingival epithelial cells

The purpose of this study was to investigate the mechanism(s) of interleukin (IL)‐8 suppression by Treponema denticola , one of the major periodontal pathogens, in gingival epithelial cells. Immortalized human gingival epithelial HOK‐16B cells were infected with wild‐type (WT), dentilisin‐deficient (K1) or flagellin‐deficient ( flgE ) T. denticola in the presence or absence of 2% human serum for 24 h. The levels of IL‐8 expression were measured with real‐time reverse transcription PCR and ELISA. In the absence of human serum, the WT and flgE , but not K1, substantially reduced not only the levels of IL‐8 protein but also of IL‐8 mRNA. Such downregulation of IL‐8 mRNA was independent of bacterial invasion. Degradation of cytokine mixture by the WT, K1 and flgE revealed dentilisin‐dependent preferential degradation of tumor necrosis factor (TNF)‐α, an IL‐8‐inducing cytokine. WT and flgE significantly decreased the levels of TNFα secreted by HOK‐16B cells, suggesting modulation of IL‐8 through dentilisin‐mediated degradation of TNFα. The addition of human serum to the culture potentiated the suppressive effect of T. denticola , resulting in substantial reductions of IL‐8 and TNFα levels, even by K1. The serum‐dependent effects of T. denticola were attributed to its ability to suppress the accumulation of intracellular reactive‐oxygen species (ROS), a group of ubiquitous signaling molecules. Pretreatment with an antioxidant suppressed TNFα‐induced IL‐8 expression, confirming the role of ROS in TNFα signaling. Collectively, T. denticola targeted a key inflammatory cytokine and its signaling molecule to modulate the host innate immune response, which provides a new insight into modulation of host immunity by a periodontal pathogen.